1. Field of the Invention
The present invention relates to epoxy resin compositions suitable for semiconductor encapsulation comprising an epoxy resin, a curing agent, and an inorganic filler. The invention also relates to semiconductor devices encapsulated with these compositions in a cured state.
2. Prior Art
Epoxy resin compositions used as encapsulants for the latest low-profile packages are loaded with greater amounts of spherical fused silica for reducing moisture absorption and preventing crack formation when the package is soldered to a substrate. However, such a high loading of fused silica results in a very small coefficient of thermal expansion. When copper is used as the lead frame material, the difference between the expansion coefficient of the lead frame and the expansion coefficient of the epoxy resin composition becomes large. This results in greater stress during thermal cycling and solder reflow,,and thus a higher tendency for crack formation.
One traditional approach to this problem is to modify the expansion coefficient of the epoxy resin composition by mixing the fused silica with another inorganic filler having a large coefficient of expansion. Many patents describe the formulation of mixtures of fused silica and crystalline silica as the inorganic filler in epoxy resin compositions.
However, because the crystalline silica used in these patents is composed entirely of crushed material, crystalline silica cannot be included in the amount required in recent compositions having high loadings of spherical fused silica. As a consequence, it was impossible to lower the moisture absorption of the cured encapsulant to the desired level. The alternative is to use spherical crystalline silica. Unfortunately, spherical crystalline silica is very difficult to obtain because it cannot be industrially manufactured. While some spherical crystalline silica is naturally available, this is the product of long years of wear in streambeds. As such, its use for encapsulating state-of-the-art semiconductor devices leaves something to be desired in terms of purity. In addition, such material cannot be given a particle size distribution optimal for high loadings.